The wide range of applications in the pressure measurement field has led to the development of many different types of pressure sensors which utilize a variety of physical sensing principles (mechanical, inductive, capacitive, etc.) and embody a large number of constructive solutions.
Pressure is defined as force per surface by means of the unit 1 bar=1*10N/m=1*10 pascal. The zero point of a pressure scale is either the pressure of an absolute vacuum or atmospheric pressure.
A distinction is therefore made between the
absolute pressure: a vacuum (0 bar) is the reference point; PA1 overpressure/underpressure: atmospheric pressure of approximately 1 bar is the reference point, and 5 bar overpressure therefore corresponds to an absolute pressure of approximately 6 bar. PA1 differential pressure: indicated by the difference between two pressures measured in the same reference system. PA1 1. Maintenance indicator (optical) PA1 2. Maintenance switch (electric) PA1 3. Combination maintenance switch/indicator.
Accordingly, pressure sensors can be divided into absolute-pressure sensors, overpressure/underpressure sensors and differential-pressure sensors. Differential pressure sensors measure the difference of the pressures sensed in two separate chambers.
In absolute-pressure sensors one of the two chambers is evacuated so that the measured pressure corresponds to the absolute pressure.
Overpressure and underpressure sensors require only one pressure chamber since the comparison takes place with respect to the external atmospheric pressure. Sensors of this type can therefore be implemented more easily than the other two types of sensors.
Because of the vacuum chamber, absolute pressure sensors are very costly to manufacture.
In addition to being a function of the type of sensing, the cost of producing a pressure sensor also depends on the precision, the measuring tolerance and the magnitude of the pressure to be measured.
Because they are simpler to produce, there are numerous overpressure and underpressure sensors. They are used where the measurement can be made with reference to the atmosphere, as, for example, for determining the degree of contamination of air and oil filters in vehicles.
In course of use, filters will become increasingly dirty. This results in an increase of the pressure drop between the dirty side (in front of the element) and the clean side (behind the element). When the pressure drop becomes too large, the filter element has to be replaced.
On the other hand, it can be quite difficult to determine the pressure drop of filter elements in hydraulic and pneumatic systems. Since, depending on the system, different operating pressures will exist here, the measurement cannot be made with reference to the atmosphere; instead the differential pressure must be sensed. The high operating pressures present another difficulty. This is because the high operating pressures make it very difficult to establish a connection from the pressure chambers to the outside.
There are basically three types of filter maintenance transducers or sensors:
Depending on the type of switching, the switches are divided into break contact switches, make contact switches and change-over contact switches.
Switch arrangements comprising a piston and a restoring spring are known. However, because the piston must always have a certain play in order to move, such switches are subject to the problem that a short circuit or bypass may occur around the piston. The medium can therefore circumvent the filter element and pass directly from the dirty side to the clean side.
In a hydraulic system, this is not so critical. In systems in which super-clean air is to be produced by means of an oil-separating air filter, this may drastically impair the efficiency of the filter.
In order to permit movement while maintaining a simultaneous seal, pistons are therefore frequently used in conjunction with membranes.
However, the conventional membranes usually allow only small strokes. Therefore, rubber bellows were frequently used for sealing which, however, cause an undefinable power shift of the pressure, which makes it difficult to achieve reproducible pressure sensing.
In oil-separating air cleaner elements, it is also insufficient to use a piston in combination with O-rings as a moving seal.
In contrast, a rolling membrane, as a special form of membrane, permits a relatively large stroke with a reproducible stroke movement and a minimum housing diameter.